GEOLOGICAL SUEVEY OF THE TERRITORIES. 137 



About twelve miles soutliwest of Denver, between Turkey and Bear 

 Creeks, are some remarkable soda lakes, which are of unusual interest. 

 They are the property of Dr. Burdsall, of Denver, in whose company I 

 made as careful an examination of them as my time would permit. There 

 are four of these little lakes, and are all located on middle cretaceous 

 rocks. The principal one lies just east of a low rounded ridge of creta- 

 ceous shale, 'No. 3, and is surrounded on the other sides by low ridges of 

 superficial sand and gravel. A little west of this cretaceous ridge there 

 is a lake, a fourth of a mile in length, but on account of the springs flowing^ 

 into it from the sloping sides of the sandstone ridge 2*fo. 1 the water is not 

 strong. The black shales of ISTo. 2, cretaceous, underlie this lake. The 

 soil for twenty feet in depth is fully impregnated with the soda; and on 

 the surface of one of the lakes is a crust which looks like dirty ice. A 

 shallow ditch which Dr. Burdsall has made out into the lake a few feet, 

 has a deposit of sulphate of soda at the bottom in a partially cr^^stalline 

 state, one and a half inches thick. Three and a half barrels of the water 

 make one barrel of the sulphate of soda, and three pounds of the soil, well 

 leached, makes one pound of the salts. The salt, by analysis, contains 

 sixty-three j)er cent, of the soda, and the water about thirty-three per cent. 

 It contains carbonate of soda, sulphate of soda, chloride of sodium, sul- 

 ]ihide of calcium, and a trace of magnesia. It Avould seem that these 

 deposits of soda must at no distant period play an important part in the 

 industrial operations of Colorado. These soda salts can be manufac- 

 tured into bicarbonate of soda, can be used in refining gold and silver, 

 also for the mtoufacture of glass with silicic acid. There is an unliin-ited 

 amount of soda at this locality, and it can be procured at a mere nominal 

 cost. 



Within a f«w yards of these lakes, and located in the black, shaly 

 clays of cretaceous formation, No. 2, are considerable quantities of brown 

 iron ore of superior quality — -as good as the best observed in the Boulder 

 coal strata. It occurs in the form of concretions, and occupies a very 

 limited area. 



CHAPTER II. 



rpOM DENVER TO COLORADO CITY. 



The city of Denver is located on the tertiary rocks which contain the 

 coal beds of the west, about ten to fifteen miles from the base of thr 

 mountains. The surface is so thickly covered with superficial drift de- 

 posits that the basis rocks are seldom seen ; but we have every reason 

 to suppose that the same beds of coal that are exposed by the uplifting 

 of the formations along the immediate flanks of the mountains, extend 

 eastward into the plains, and of course underlie, at certain depths, the 

 city of Denver. 



As we pass southward, up the valley of the South Platte, we find the 

 tertiary sandstones exposed occasionally in the banks of the river; and. 

 near the caiion a seam of coal has been opened and worked to some 

 extent. The tertiary beds extend quite close up to the foothills of the 

 mountains, leaving a comparatively narrow space for the exhibition of 

 the older, unchanged rocks. Still, we may walk across the upturned 

 edges of them all and study them with care. 



The valley of the South Platte presents a fine display of the terraces ; 

 and the drift, filled with water-worn rocks, is very thick. The sand- 

 stones of the tertiary formation are also plainly seen, appearing to be 



